In these years, with the increasing demand for the miniaturization, multifunctionalization and cordless operation of electronic devices, the development of high performance electrochemical elements has been actively promoted. Examples of such electrochemical elements include secondary batteries repeatedly usable by being charged and capacitors. Examples of the secondary batteries include nickel-cadmium secondary batteries (NiCd batteries) obtained by using cadmium, nickel-hydrogen secondary batteries (Ni-MH batteries) obtained by using hydrogen storage alloys and nonaqueous electrolyte secondary batteries (lithium ion batteries) using lithium compounds. Examples of the capacitors include redox capacitors and electric double layer capacitors.
Among these batteries, the Ni-MH batteries and the lithium ion batteries have a structure in which an electrode group fabricated by interposing separators between positive electrodes and negative electrodes is housed together with an electrolyte in a container.
The positive electrode of the Ni-MH battery is formed by binding together nickel hydroxide or nickel oxyhydroxide as an active material, carbon as a conductive material, a cobalt powder as an additive and the like to each other with a binder to prepare a mixture, and further by binding the bound mixture to a metal current collector. On the other hand, the negative electrode is electrically connected to a hydrogen storage alloy, and is prepared by coating a metal current collector such as a punching metal, a metal porous plate, a foam metal plate or a net-like metal fiber sintered plate with a paste prepared by kneading a conductive material composed of a nickel powder and the like and a binder in the presence of water and by drying the coated paste. For the electrolyte of a Ni-MH battery, an aqueous solution of a strong alkali such as potassium hydroxide is used.
The positive electrode of a lithium ion battery is prepared as follows: a mixture is prepared by adding a conductive material such as a metal powder or carbon and a binder to an active material such as lithium cobaltate; the mixture is kneaded in the presence of N-methyl-2-pyrrolidone or the like to prepare a paste; the paste is applied with a doctor blade to a metal current collector and dried; and thus the positive electrode is prepared, wherein the binder binds the active material such as lithium cobaltate and the conductive material to each other in the paste and the paste is bound to the metal current collector. On the other hand, the negative electrode is prepared as follows: a mixture is prepared by adding a binder to a carbon material as an active material; the mixture is kneaded in the presence of water or the like to prepare a paste; the paste is applied with a doctor blade to a metal current collector and dried; and thus the negative electrode is prepared, wherein the binder binds the carbon material to the metal current collector. In the electrolyte of a lithium ion battery, a nonaqueous solvent such as propylene carbonate is used, and usually a supporting electrolyte salt is added to the electrolyte.
In general, an electrode of a capacitor includes a current collector sheet formed of aluminum, stainless steel or the like and an electrode layer formed on the surface of the current collector sheet. The electrode layer is formed of a mixture composed of an active material having a high specific surface area such as activated carbon, a conduction aid such as conductive carbon and a binder. The electrode layer is formed by applying a coating liquid composed of the active material, the conduction aid and the binder to the surface of the current collector sheet. Alternatively, the electrode layer is formed by forming a sheet formed of a mixture composed of the active material, the conduction aid and the binder, and by adhering the mixture sheet to the surface of the current collector sheet. In the electrolyte of a capacitor, a nonaqueous solvent such as propylene carbonate or an aqueous solution such as a sulfuric acid aqueous solution is used, and usually a supporting electrolyte salt is added to the electrolyte.
Accordingly, a binder to form these electrodes is required to be: (1) excellent in the corrosion resistance against the electrolyte, (2) high in any of the binding property between the current collector and the active material, the binding property between the conductive materials, and the binding property between these individual materials, (3) stable under the sever environment such that a voltage is exerted to the binder in the battery and (4) low in the internal resistance and able to maintain a high conductive property when used to form an electrode. When such electrodes that meet these requirements are used as the electrodes in a rechargeable battery, the cycle properties of the rechargeable battery is improved, and when such electrodes that meet these requirements are used in a capacitor, the heat degradation resistance of the capacitor is improved.
For the purpose of meeting these requirements, the following techniques have been proposed.
JP-A-9-251856 discloses in [0007] thereof a method in which a self-emulsifiable polyolefin emulsion containing no surfactant is used as a binder for electrode formation. The self-emulsifiable polyolefin emulsion is an emulsified aqueous solution prepared by introducing carboxyl groups into the olefin skeleton in polyethylene, polypropylene or the like, by water-solubilizing the thus obtained polyolefin with an alkali such as ammonia, alkanolamine or caustic soda and additionally by highly dispersing the polyolefin for emulsification ([0008]).
JP-A-2005-63735 discloses a binder for secondary batteries including a polyolefin resin that contains an unsaturated carboxylic acid component, an ethylene component, an acrylic acid ester or methacrylic acid component. In [0020] of JP'735, presented is a method for preparing an aqueous dispersion without using any nonvolatile emulsifying agent.
JP-A-8-50894 discloses a technique in which an aqueous dispersion of a polyolefin resin having an average particle size of 20 μm or less is used as a binding agent. Claim 2 of this patent document presents, as an example, an aqueous dispersion of a thermoplastic elastomer including an ethylene-propylene copolymer.
JP-A-2002-251998 discloses an electrode binder that includes an amorphous polypropylene homopolymer or an amorphous copolymer that is a copolymer between propylene and an olefin having 2 to 8 carbon atoms wherein the content of propylene is 50% by mass or more. In [0019] of JP'998, presented are a method in which a binder is used as a solid substance as it is, a method in which a solid substance is dissolved in an organic solvent to be used and a method in which a solid substance is used as an emulsified substance.
JP-A-7-161348 discloses a technique in which an aqueous dispersion of a polyolefin resin is used at the time of preparing the negative electrode mixture. In of this patent document, disclosed as the polyolefin resin are polyethylene, polypropylene, poly-1-butene, polymethylpentene, and modified polyolefins obtained by copolymerizing other monomers; and the presented examples of the other monomers are acid components such as acrylic acid, the salts of the acid components, acrylic acid esters, methacrylic acid esters and vinyl acetate.
WO2004/104090 discloses a polyolefin resin aqueous dispersion including a polyolefin resin that contains 50 to 98% by mass of an unsaturated hydrocarbon having 3 to 6 carbon atoms and 0.5 to 20% by mass of an unsaturated carboxylic acid unit and a basic compound having a boiling point of 185° C. or lower, but substantially not including any water-compatibilizing agent having a boiling point of 185° C. or higher, wherein the number average particle size of the polyolefin resin in the aqueous dispersion is 1 μm or less.
JP-A-11-162794 and JP-A-2000-208368 each describe the use, as a binder, of a styrene-butadiene polymer having a specific composition. JP-A-2001-307965 proposes the use, as a binder, of a mixture composed of a styrene-butadiene polymer and a cellulose polymer.
However, the techniques described in the afore-mentioned individual documents have the following problems.
According to the results obtained by the present inventors' investigation of the self-emulsifiable polyolefin emulsion disclosed in JP-A-9-251856, the introduction of carboxyl groups into the olefin skeleton does not necessarily result in meeting the performance required for a binder. For example, some types of olefin resins have proved to be poor in the binding property with conductive materials, indicating that no involvement of an emulsifying agent necessarily leads to a satisfactory performance.
As for JP-A-2005-63735, the binding properties are demanded to be more improved such that a further smaller amount of the binding agent will be sufficient for successful binding.
As for JP-A-8-50894, ethylene-propylene copolymer has no ionizing functional groups such as carboxylic acids, and hence an emulsifying agent is essential for preparation of an aqueous dispersion of this copolymer. Usually, for the purpose of emulsifying such a resin having no ionizing functional groups, an emulsifying agent or the like is used for forcibly emulsifying the resin, but it is difficult to make small the particle size. Consequently, the binding agent amount required for binding a certain amount of an active material is increased to cause increase of the internal resistance of the electrode. Additionally, when an emulsifying agent is used in a binding agent, the emulsifying agent migrates to the interface associated with the active material or the electrode to degrade the binding properties, and accordingly offers problems associated with long-term usability and the cycle property.
When, as in JP-A-2002-251998, a binder resin is used as it is in a solid state, the binder undergoes an insufficient mixing performance with the active material as a coating material for electrode formation, and a thin film formation is difficult with a method involving kneading and extruding. When such a binder is dissolved with an organic solvent, recovery of the solvent is needed and adverse effects may be exerted on the environment. Additionally, when the solution, the active material and the like are mixed together, the solution uniformly covers the whole active material to increase the amount of the binder required for binding. Consequently, the internal resistance of the battery is increased to degrade the performance of the battery. On the other hand, a method in which a binder is converted to an emulsified substance can be regarded as suitable for a binder for electrode formation. However, disclosed in JP'998 is only a method for which a nonvolatile water-compatibilizing agent such as a surfactant or a carboxy-modified wax is essential although it is stated that suitable is a binder in which an emulsifying agent or a dispersant is not used or used as little as possible; additionally, presented in Examples in JP'998 is only a method in which the binder is used as a solid substance.
In JP-A-7-161348, no description is presented on the proportions of the acid component, the component of a salt of the acid component and the component of an ester of the acid component in relation to the copolymerization component. In general, the effects of the type and composition of the copolymerization component on the binding properties of the resin and the water-compatibilization of the resin are significant. However, in JP'348, the specific presentation in Examples is restricted to a homopolymer such as polyethylene or polypropylene containing no acid component and to an example in which ethylene is copolymerized with an acid component and a salt of the acid component. Additionally, as a preferable modified polyolefin, only a modified polyethylene is quoted.
The application of the aqueous dispersion liquid disclosed in WO2004/104090 is mainly the usage as adhesives for films and coating materials. This document does not contain any description on the properties as the binder for electrode formation.
The binders described in JP-A-11-162794, JP-A-2000-208368, and JP-A-2001-307965 are disadvantageously poor in heat resistance. Specifically, the activated carbon used as an electrode material is large in specific surface area, and accordingly tends to absorb moisture; therefore, the slurry applied onto the current collector is needed to be dried at a high temperature to remove the moisture in the slurry. However, when a styrene-butadiene polymer is used as a binder, disadvantageously the styrene-butadiene polymer loses its flexibility by high-temperature drying and the delamination from the current collector occurs to increase the internal resistance. When a capacitor is actually fabricated by using this binder, disadvantageously the internal resistance is high and accordingly the heat degradation resistance is low.
The present invention has been achieved for the purpose of solving the afore-mentioned problems and an object of the present invention is to provide a binder for electrode formation excellent in cycle properties when used in batteries and excellent in heat degradation resistance when used in capacitors by improving the corrosion resistance against the electrolyte, the binding properties, the stability and the internal resistance.